This invention relates to cargo vehicles, such as rental trucks, of the type which are unloaded from the ground, as opposed to a loading dock, for example, when moving one's household furniture and, more particularly, to a pneumatic or air suspension therefor having a control system which provides for operator actuated lowering of the cargo bed to the minimum height above the ground to facilitate loading and unloading of the vehicle while automatically preventing operation of the vehicle with the cargo bed in the lowered position. Such a system would be used in place of hydraulic or electric lift attachments which may be expensive and improperly used by inexperienced operators.
Pneumatic suspension systems for larger vehicles such as tractor trailers, are well known and usually include one or more air springs at each end of each axle where the sprung mass (chassis, body and load) of the vehicle is supported on the unsprung mass (the wheels, suspension and axles) of the vehicle. Typically, the source of air pressure for such air suspension systems is the air brake system of the vehicle, including the air brake supply reservoir of the vehicle. It is known to allow for selective inflation or deflation of the air springs or air bags for raising and lowering the rear cargo area of the vehicle to align it with a loading dock to facilitate loading and/or unloading thereof. Conventionally, such systems utilize manually controlled valve means which are disposed at the rear of the vehicle so that the operator quickly can ascertain the position of the rear cargo area and control its position to achieve the desired elevation relative to the dock. This requires the presence of an operator at the rear of the vehicle as well as controls to facilitate adjustments to dock height.
Many vehicles are provided with automatic levelling systems for air suspensions. It is common in the art to provide override means for controlling the elevation of a vehicle relative to the axles of the vehicle. Such systems may include a regulating valve having an arm pivoted thereto and a fixed length link extending between the arm and the axle. As one end of the axle moves downwardly relative to the vehicle bed when the vehicle tilts, the link moves downwardly as does the arm on the regulating valve. The movement of the arm causes the valve to add or bleed pressurized air from the air bags effectively to position the vehicle bed to a condition whereat the vehicle bed is generally parallel to the axle and hence compensate for downward movement of the axle. Such alterations in clearance height between the sprung and unsprung masses tend to maintain the sprung mass generally parallel to the unsprung mass even though the unsprung mass moves upwardly or downwardly through a horizontal plane. U.S. Pat. No. 3,074,739 provides means selectively to override the levelling system of the type described by replacing the link in the automatic levelling system with a spring biased pneumatic cylinder which may be pressurized at will to override the automatic system. However, such type of patented system is complex, expensive and relies on springs as well as compressed air to operate and does not provide the precision control required. U.S. Pat. No. 4,335,901 provides a system which operates essentially independently of the vehicle's compressed air system although making use of such compressed air system to pressurize the air bags, if required For the link of U.S. Pat. No. 3,074,739, a cylinder and piston actuator is used which is connected to a master control via a conduit, the actuator chamber, the conduit and the master control chamber being filled with a compressible fluid. The master control chamber is varied in volume to produce a corresponding change in the actuator chamber resulting in relative movement between the actuator chamber and the piston, operating the regulating valve. The master control manually is operated selectively to effect pressurization or bleeding of the air bags to raise or lower the vehicle bed. The system taught by said last mentioned patent does not provide any suggestion for automatic reinflation of the air bags so that the vehicle cannot be driven with the air bags deflated. Further, no provision was made therein for automatic reset of the control system when the vehicle is placed in drivable condition. While a simpler and less expensive system than that of U.S. Pat. No. 3,074,739, the system of U.S. Pat. No. 4,335,901 yet was neither simple nor economical.
As a proposal for guarding against the operation of a deflation system while same is in the elevating or deflating condition, U.S. Pat. No. 4,558,886 provides a safety interlock in the air brake system of the vehicle but does not provide means for automatically reinflating the air bags when the loading or unloading is completed and before the vehicle is moved.
Another suggestion made in the prior art for providing a system for raising and/or lowering the front and/or rear of a vehicle having an air-bag suspension system can be found in U.S. Pat. No. 4,341,398 wherein a height-control valve was provided between the air reservoir and the air bags located at the end of the vehicle intended for selective raising and/or lowering and a by-pass valve is provided functioning to by-pass the height control valve. The by-pass valve is solenoid operated and is controlled by a sensing switch providing a return to the normal level when lowering to the desired level has been effected by operating selected exhaust valves operating upon the individual air bags concerned. While the operation suggested by the last mentioned patentee is driver controlled, there are no suggestions therein how automatically to assure that the air bags are reinflated before the vehicle is driven.